Stepped capacity control
A maximum number of four compressors are managed, with a maximum of four capacity control steps each. Capacity control is achieved by three relay outputs which, when suitably commanded, short-circuit the refrigerant thrust by the compressor, varying its capacity and, therefore, the power input into the circuit.
Configuration of stepped capacity control relays
The enabling sequence of the capacity control relays differs for each compressor. Therefore, the software has a facility for configuring the enabling sequence according to the needs of different compressor manufacturers. For multi-card systems: as several compressors are housed on the same machine, it is considered that the compressors controlled by each pCO are perfectly equal and, therefore, the capacity control configuration selected on board the master card also applies to the slave cards. The following table shows examples of the configuration of the dedicated digital outputs for the different power stages entered.
The effective status of the digital output is indicated. The relation between the data in the table and the values set on the display. Closed = ON Open = OFF
Default configuration :
% LOAD Relay 1 Relay 2 Relay 3
25% CLOSED OPEN OPEN
50% OPEN OPEN CLOSED
75% OPEN CLOSED OPEN
100% OPEN OPEN OPEN
Configuration example :
% LOAD Relay 1 Relay 2 Relay 3
25% OPEN CLOSED CLOSED
50% CLOSED CLOSED OPEN
75% CLOSED OPEN CLOSED
100% CLOSED CLOSED CLOSED
Stepped capacity control times
Delays are specified for capacity control management. These delays can be set when the capacity controls are enabled. Such delays indicate the minimum operating time of a compressor at a given power stage. If the machine is enabled at maximum level request, these delays prevent a changeover from power level 0 to maximum level.
Graph of times for capacity control in 4 steps:
T1 T2 T3 25% 50% 75% 100% T4
With Bitzer compressor the time T2-T3-T4 are force the follow value: T2: The minimum value is force to 10 second
T3: zero T4: zero
The T1 time does not have restriction.
To have more flexible management of the unloader during the unit start up and the normal working condition, it is possible to set a time (mask Te) starting from the pump on and after this the unit is in normal working mode. If this time equal to zero then the management is disable.
During start up mode the time T1, T2, T3 (mask T7) are respected, but in normal mode they are ignored and an equal time for all the uloaders is used set by mask Te. With Bitzer compressor the minimum time is force to 10 second.
Special management of capacity control first stage
A facility is provided for enabling special management of the first stage of capacity control, managing the compressor's special requirements when it is operating at low power. In general, the control entails the use of the first capacity control stage only at power-up and if temperature falls below the control set-point. When controlling the compressor, this type of control uses a reduced power modulation range, between the second and maximum power stages.
Management varies according to whether the compressor is in its starting or disabling stage. In both cases, you are recommended not work at 25% power for too long. • Starting: after being started, if the compressor does not receive any thermostatic request for changeover to the second capacity control stage, the changeover
is forced by the software after a time which can be set on the screen (T1).
• Power-down: if a reduction in the power of the circuit is requested, power is controlled between the maximum and second capacity control stage. Only if temperature drops below set-point value, the compressor is forced to operate according to the first capacity control stage for the set time (T1).
This special operating mode is enabled from the screen. If the first capacity control step is not enabled, it is treated as just any step. The compressor can operate at this power level for an infinite time.
Standard Chiller/HP modulare per compressore a vite Generico/Bitzer con driver CAREL
Stepped capacity control with control at inlet
A description of stepped capacity control of 4 compressors with four capacity control steps each:
Setpoint di regolazione Temperatura ingresso evaporatore Banda di regolazione C4P1 P2 P3 P4 C1P1 P2 P3 P4 C2P1 P2 P3 P4 C3P1 P2 P3 P4
All compressors and the relevant capacity control steps will be proportionally positioned in the band. Increasing temperature values will cause the control steps to be subsequently input. Each step will be input according to the set delay times. The compressors will be started at the first entered capacity control stage. If special management of the first capacity control stage was selected, control will be effected according to the description in the dedicated section. In any event, the times for the capacity controls will be applied as described.
Stepped capacity control with control at outlet
A description of stepped capacity control of 4 compressors with four capacity control steps each: Activation of compressors
if the water temperature measured by the probe located at the evaporator outlet exceeds the threshold of Control Set-point + Control Band (Point B), the number of power stages will be increased - the power stages were input according to the set parameter known as "delay between power-up of different devices".
Temperatura Uscita Evaporatore Punto B Setpoint di regolazione C4P1 P2 P3 P4 C1P1 P2 P3 P4 C2P1 P2 P3 P4 C3P1 P2 P3 P4 Tempo [s] Ritardo Attivazione Dispositivi
In this configuration the time between the activation of the steps will be equal to the set time between the starts of different compressors, while in the event of capacity-control, the delay time between load steps set will still be applied, and therefore the higher of the two times will prevail.
Power-down of compressors
If the water temperature measured by the probe located at the evaporator outlet falls below the Control set point (Point A), then the number of load steps will be decreased, according to the parameter “device deactivation delay”.
Ritardo Disattivazione Dispositivi Soglia spegnimento forzato Temperatura Uscita Evaporatore Setpoint Regolazione Punto A C4P1 P2 P3 P4 C1P1 P2 P3 P4 C2P1 P2 P3 P4 C3P1 P2 P3 P4 Tempo [s]
If the temperature falls below the forced off threshold, the compressors are stopped irrespective of the set delays, to avoid the activation of the antifreeze alarm.
Continuous capacity control
A maximum number of four compressors are managed, with continuous capacity control. The compressor's capacity is controlled by two relay outputs, which, when suitably controlled, enable compressor power to be increased or reduced, varying the capacity of the compression chamber. Compressor power is controlled by sending impulses to the outputs of the capacity control relays. These impulses command the compressor to be charged or discharged. These impulses are at a constant frequency, settable, and of variable duration between two minimum and maximum limits, also settable. As there is no acquisition regarding the absolute position of the compressor 's capacity control valve, and, consequently, as no direct verification is possible of the power percentage input in the circuit, a time based control is run. With this control, when a set time threshold is reached, the compressor is considered fully charged/discharged and thus control of the capacity control impulses is suspended.
Durata Impulso Calcolata
Periodo Impulso Periodo Impulso Periodo Impulso Durata Impulso Calcolata Durata Impulso Calcolata Durata Impulso Calcolata Tempo [s]
13.4.1 Configuration of continuous capacity control relays
The control method of the capacity control relays differs for each compressor. Therefore, the software has a facility for configuring the enabling sequence according to the needs of different compressor manufacturers.
For multi-card systems: as several compressors are housed on the same machine, it is considered that the compressors controlled by each pCO are perfectly equal and, therefore, the capacity control configuration selected on board the master card also applies to the slave cards. The following table shows examples of the configuration of the dedicated digital outputs for the different power stages entered.
The effective status of the digital output is indicated.
The relation between the data in the table and the values set on the display. Closed = ON
Open = OFF
Default configuration :
Compressor behaviour Relay 1 Relay 2
Power reduction CLOSED CLOSED
Power stand-by OPEN CLOSED
Power increase OPEN OPEN
The power stand-by configuration is taken on by the outputs when no variation of input power is requested, or if the maximum/minimum compressor power is reached, or because the water temperature measured by the probe located at evaporator outlet is inside the neutral control zone. For compressor charging /discharging, the digital outputs of the pCO card are commanded alternately according to the stand-by and charge/discharge configuration, causing the dedicated relay to pulse.
13.5 Continuous capacity control with control at outlet
Temperature control with compressors on continuous capacity control can occur only if control at outlet is selected, according to the temperature values measured by the probe located at evaporator outlet. To that end, further configuration parameters are input. They are specific for the particular type of compressor, and are added to those previously mentioned in the description of the special type of control.
Employed Parameters:
• Neutral zone for continuous capacity control • Impulse period
• Charging impulse minimum duration • Charging impulse maximum duration • Discharging impulse minimum duration • Discharging impulse maximum duration • Forced discharge period at compressor power-up
• Capacity control relay forcing enabled when compressor is OFF: Outputs used :
• Compressor capacity control Relay 1 • Compressor capacity control Relay 2
A Soglia Spegnimento Forzato
D Banda di regolazione per controllo in uscita
E Temperatura Uscita Evaporatore Accensione compressori Incremento modulato potenza Spegnimento compressori Decremento modulato potenza C Setpoint Regolazione B Zona neutra
13.5.1 Control of continuous capacity control according to points in the graph
According to the set-point values, the control band with control at output and the neutral zone of compressors on continuous capacity control, points C, D and E are identified. If the water temperature measured by the probe located at evaporator outlet exceeds point E
Point E = Control set point + Control band/2 + Dead zone /2
Then there will be a request for the compressor to start and an increase in capacity according to charge impulses of the maximum duration until reaching the maximum compressor load time. If the water temperature measured by the probe located at evaporator outlet is below point B
Point B = Control set point + Dead zone /2 - Control band/2
In this case, there is a request for the compressors to be discharged according to the maximum duration impulses until compressor maximum discharging time is reached and until possible power-down. If the water temperature measured by the probe located at the evaporator outlet is between points D-E/B-C
Point D = Control set point + Dead zone Point C = Control set point
Then the power of the compressor will be increased/reduced by impulses of variable duration according to the values calculated within the minimum and maximum limits set for an infinite time.
Standard Chiller/HP modulare per compressore a vite Generico/Bitzer con driver CAREL
13.5.2 Power-up of compressors (temperature above point E)
The compressors are powered up in sequence at a rate calculated by the set time required to reach maximum power. As there is no absolute measurement of the effective capacity, when the compressor is started it performs a forced unload cycle for a set time (unloader relays energised continuously according to the unload configuration). Subsequently, the compressor power will be increased by maximum duration impulses.
13.5.3 Increase of compressor power
When the maximum time limit for reaching maximum power is reached, a forced charging cycle is commanded for a time of 20% of the set threshold, then the compressor capacity control relays change to the power stand-by configuration.
If the temperature remains in the power-up zone (beyond point E), every ten minutes a forced charging cycle is commanded with a duration of 20% of the time required to reach the maximum set power.
In the case of multi-compressor units, the periodic forced charging cycle will be carried out by all powered-up compressors which have reached maximum power. 13.5.4 Modulated increase of power (temperature in range between points D-E)
The compressor's power is modulated in this temperature range, by applying charging impulses of variable duration to the capacity control relays (duration is calculated between the minimum and maximum values set according to the measured temperature values).
For multi-compressor units, modulated increase of power will occur simultaneously for all powered-up compressors. 13.5.5 Operation of compressor in neutral zone (temperature in range between points C-D)
If the temperature value locates inside the neutral zone, the capacity control relays of all powered-up compressors change to the power stand-by configuration, thus maintaining the power level that had been reached.
13.5.6 Modulated reduction of power (temperature in range between points C-B)
The compressor's power is modulated in this temperature range, by applying discharging impulses of variable duration to the capacity control relays (duration is calculated between the minimum and maximum values set according to the measured temperature values). For multi-compressor units, modulated reduction of power will occur simultaneously for all powered-up compressors.
13.5.7 Power-down of compressors (temperature below point B)
The compressors are first unloaded by sending unload impulses of the maximum duration to the unloader relays. The compressors are then powered down, by reducing the number of requested devices, at a rate equal to the time required to reach minimum set power.
FIFO Rotation is applied, whereby the first powered-up compressor is discharged and then powered-down. Instead, if rotation is disabled, the last powered-up compressors is discharged and then powered down.
13.5.8 Derivative regulation in the increasing zone
In the increasing zone, Tb mask, comes controlled every “Derivative Time” if the outlet temperature is changed in order more than 0,2 °C. If this is true the compressor remains in stand-by until the new control. This management can be disabled with “Derivative time” equal to 0.